Although antibiotics have been repeatedly evaluated as adjuncts to peridontal therapy, the results in adult peridontitis are unimpressive. Interpretation of results has been complicated since previous investigations have used probing depths as the primary indicator of success. This study will evaluate whether adjunctive antibiotic therapy alters the recolonization of the microbiota following conventional peridontal therapy. The ability to alter the recolonization and regrowth of the subgingival microbiota of a peridontal pocket should prolong the interval for routine maintenance therapy and would thereby offer a substantial therapeutic advantage. Treatment and maintenance care would be less labor intensive, less costly and more acceptable to the patient. The study will evaluate the effect of routine periodntal therapy (scaling, root planing, oral hygiene instructions) with and without the administration of the systemic antibiotic tetracycline, upon the microbiologic parameters of the disease, and to describe the longitudinal events in the recolonization of the pocket microbiota in a human population of 20 patients exhibiting moderate to severe adult peridontitis. Two selected oral sites from each category representing moderate and severe disease will be subjected to a baseline examination including: plaque index, gingival index, probing depth, probing attachment level, standardized radiographs, and microbiological characteristics of subgingival plaque. Supragingival and subgingival scaling and root planing will be performed on all patients. Patients will then be randomly divided into two groups, one group will receive tetracycline and the second a placebo. Oral hygiene instruction and polishing will be performed at monthly appointments, but no further subgingival treatment will be administered. Registrations and sampling will be repeated at 3 weeks, 3 months, and 6 months following active therapy. This study will evaluate conventional scaling and root planing and scaling and root planing with tetracycline. Evaluation of the use of tetracycline and its effect upon recolonization of a periodontal pocket and subsequent progression of peridontal disease should provide new knowledge as to the efficacy of the adjunctive use of an antibiotic with routine scaling and root planing in cases of adult peridontitis.